Well it's a bit difficult I suppose, but you might have noticed the username and password are sent over the wire using base64 encoding. Not really secure, unless you have SSL in place.

Digest

Digest is designed to be more secure. The password is never sent over the wire in plain text, but rather as a hash. The implications of the usage of a hash is that it can never be decrypted. We can only validate the hash by applying the same hash function to the password we have. If the hashes match, the password was correct.

Information coming from the server:

|realm |A string which will be used within the UI and as part of the hash.
|qop |Can be auth and auth-int and has influence on how the hash is created. We use auth.
|nonce |A unique code, which will be used within the hash and needs to be sent back by the client.
|opaque |This can be treated as a session id. If this changes the browser will deauthenticate the user.

Information from the client:

|username |The supplied username
|realm |Same as server response.
|nonce |Same as server response.
|uri |The authentication uri
|response |The validation hash.
|opaque |Same as server response.
|qop |Same as server response.
|nc |Nonce-count. This a hexadecimal serial number for the request. The client should increase this number by one for every request.
|cnonce A unique id generated by the client

So how do we know if the password was correct? We van validate using the following formula (pseudo code).

<?php$realm='The batcave';// Just a random id $nonce=uniqid();// Get the digest from the http header $digest=getDigest();// If there was no digest, show login if(is_null($digest))requireLogin($realm,$nonce);$digestParts=digestParse($digest);$validUser='admin';$validPass='1234';// Based on all the info we gathered we can figure out what the response should be $A1=md5("{$digestParts['username']}:{$realm}:{$validPass}");$A2=md5("{$_SERVER['REQUEST_METHOD']}:{$digestParts['uri']}");$validResponse=md5("{$A1}:{$digestParts['nonce']}:{$digestParts['nc']}:{$digestParts['cnonce']}:{$digestParts['qop']}:{$A2}");if($digestParts['response']!=$validResponse)requireLogin($realm,$nonce);// We're in! echo'Well done sir, you made it all the way through the login!';// This function returns the digest string functiongetDigest(){// mod_php if(isset($_SERVER['PHP_AUTH_DIGEST'])){$digest=$_SERVER['PHP_AUTH_DIGEST'];// most other servers }elseif(isset($_SERVER['HTTP_AUTHENTICATION'])){if(strpos(strtolower($_SERVER['HTTP_AUTHENTICATION']),'digest')===0)$digest=substr($_SERVER['HTTP_AUTHORIZATION'],7);}return$digest;}// This function forces a login prompt functionrequireLogin($realm,$nonce){header('WWW-Authenticate: Digest realm="'.$realm.'",qop="auth",nonce="'.$nonce.'",opaque="'.md5($realm).'"');header('HTTP/1.0 401 Unauthorized');echo'Text to send if user hits Cancel button';die();}// This function extracts the separate values from the digest string functiondigestParse($digest){// protect against missing data $needed_parts=array('nonce'=>1,'nc'=>1,'cnonce'=>1,'qop'=>1,'username'=>1,'uri'=>1,'response'=>1);$data=array();preg_match_all('@(\w+)=(?:(?:")([^"]+)"|([^\s,$]+))@',$digest,$matches,PREG_SET_ORDER);foreach($matchesas$m){$data[$m[1]]=$m[2]?$m[2]:$m[3];unset($needed_parts[$m[1]]);}return$needed_parts?false:$data;}?>

As you can see we need to have a plain-text version of the password in order to validate the user. It's not a good idea to store the plain-text password, therefore it's strongly recommended to store the result of $A1 instead.

Security improvements

It's smart to validate the contents of opaque, nonce and realm. If you have the data stored on the server, why not check it.

The nc should be an ever increasing number. You could store the number and track to make sure it doesn't make any big jumps. It's not wanted to be extremely strict about the sequence, because you might miss a number, and requests could come in be out of order.

'qop' is quality of protection. This serves as an integrity code for the request. A hacker could steal all your HTTP Digest headers and simply change the body to make it do something else. If 'qop' is set to 'auth', only the requested uri will be taken into consideration. If 'qop' is 'auth-int' the body of the request will also be used in the hash. (A2 = md5(request-method:uri:md5(request-body))).